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A skin disease caused by sand flies is on the rise in the U.S.

ARI SHAPIRO, HOST:

A skin disease spread by sand flies was long considered a tropical disease, only acquired by Americans traveling abroad. Well, recent findings from the Centers for Disease Control and Prevention show that the disease is actually spreading in the southern U.S. and has been for quite some time. NPR's Pien Huang has more.

PIEN HUANG, BYLINE: Back in 2014, Dr. Bridget McIlwee saw a young patient in central Texas. He was a 3-year-old boy, and he had a growing rash on his ear.

BRIDGET MCILWEE: They looked a little bit like - almost kind of a benign mole that you would see in a child, except that you wouldn't expect something like that to come up quickly and then multiply.

HUANG: McIlwee sent a sample to a lab, and the diagnosis came back as cutaneous leishmaniasis. It's a skin disease caused by parasites spread from person to person through sand fly bites. It often resolves on its own but can be treated.

MCILWEE: And I was shocked because in medical school, we're taught that this is a tropical disease - something that you see in immigrants, military returning from deployment, people who went on vacation to South America or Asia or Africa.

HUANG: It's not what she expected to see in a child in central Texas who hadn't gone anywhere. So a few years ago, McIlwee and her colleagues wrote a paper for the journal JAMA Dermatology. It said the textbooks are wrong. Parasites that cause leishmaniasis are living and thriving in the U.S. Those findings were recently confirmed and bolstered by new research from the CDC. Vita Cama, a microbiologist with the agency, says the parasite in the U.S. is called Leishmania mexicana.

VITA CAMA: The genetic signature between mexicana on people who travel abroad and among people who did not have any international travel - there was a very clear distinction between both.

HUANG: And Cama says that specific parasite that's been spreading the disease in the U.S. has been here for a while.

CAMA: It's not a new event. This has been happening - we don't know how far back but at least since 2005.

HUANG: 2005 because that's when the CDC started DNA testing their samples. Dr. Peter Melby, an infectious diseases doctor at University of Texas Medical Branch, says it goes back further than that. In the 1990s, Melby saw a patient, a Texas rancher, who'd been misdiagnosed years before with leprosy. The patient wasn't responding to treatment because he, in fact, had leishmaniasis. Melby says the Leishmania parasites were tracked to a nest of rats living on the man's ranch.

PETER MELBY: So it's a very unique south Texas ecosystem where the sand fly and the rodent live symbiotically there in these nests.

HUANG: For decades, leishmaniasis has been circulating at what seems like low levels in south and central Texas. But in the past 10 years, Melby says the range has been spreading north.

MELBY: It has begun to emerge in northern Texas and even up into Oklahoma, in New Mexico and Arizona.

HUANG: And the type of sand fly that spreads the parasite is already found as far north as Ohio. Climate models say there are some 12 million people in the U.S. who could be getting exposed to leishmaniasis locally. That number could more than double over the next 60 years. Researchers want doctors and patients to know about the possibility, and they want to see the textbooks changed to reflect the reality that leishmaniasis is here. Pien Huang, NPR News.

(SOUNDBITE OF STORMZY SONG, "FIRE + WATER") Transcript provided by NPR, Copyright NPR.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.

Pien Huang is a health reporter on the Science desk. She was NPR's first Reflect America Fellow, working with shows, desks and podcasts to bring more diverse voices to air and online.